1. Field of the Invention
The present invention relates to a switching converter.
2. Description of the Related Art
As a backlight of a liquid crystal panel or as an illumination device, semiconductor light sources such as LEDs (light-emitting diodes) have been becoming popular. In recent years, in the field of LED illumination devices, the development of step-down switching converters has been advancing. FIG. 1 is a circuit diagram showing a step-down switching converter investigated by the present inventors. A switching converter 100r receives an input voltage VIN from an unshown power supply, and steps down the input voltage VIN thus received, so as to output an output voltage VOUT to an LED light source 502 that functions as a load. Furthermore, the switching converter 100r stabilizes a current (which is referred to as a “load current” or “driving current”) that flows through the LED light source 502 to a target value IREF. For example, the LED light source 502 is configured as a light-emitting diode (LED) string. The switching converter 100r sets the target current value IREF of the load current ILED according to a target luminance set for the LED string.
The switching converter 100r includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a smoothing capacitor C1, a rectifier diode D1, a switching transistor M1, an inductor L1, an auxiliary winding L2, and a detection resistor RCS.
In the on period of the switching transistor M1, a current that flows through the switching transistor M1 also flows through the detection resistor RCS. A voltage drop (detection voltage) VCS across the detection resistor RCS is fed back to a current detection (CS) terminal of the control circuit 200r. 
The control circuit 200 includes a current limit comparator 202, a zero current detection circuit 204, a logic circuit 206, and a driver 208.
FIG. 2 is an operation waveform diagram showing the operation of the switching converter 100r shown in FIG. 1. During a period in which the switching transistor M1 is turned on (on period), the coil current IL corresponds to a current IM1 that flows through the switching transistor M1, which flows through the LED light source 502, the inductor L1, the switching transistor M1, and the detection resistor RCS. As the coil current IL increases, the current detection signal VCS rises. The current limit comparator 202 compares the current detection signal VCS with a target voltage VADIM that is set according to the target current value IREF. When the current detection signal VCS reaches the target voltage VADIM, i.e., when the coil current IL reaches a limit current ILIM (=VADIM/RCS), a limit current detection signal S1 is asserted (e.g., set to high level). In the on period, the energy stored in the inductor L1 increases.
When the limit current detection signal S1 is asserted, the logic circuit 206 switches a pulse signal S2 to an off level (e.g., low level) corresponding to the off state of the switching transistor M1. The driver 208 turns off the switching transistor M1 according to the pulse signal S2.
During an off period of the switching transistor M1, the coil current IL corresponds to a current ID1 that flows through the rectifier diode D1, which flows through the LED light source 502, the inductor L1, and the rectifier diode D1. With the passage of the off time, the energy stored in the inductor L1 decreases, which decreases the coil current IL.
The auxiliary winding L2 is coupled with the inductor L1, which forms a transformer T1. A voltage VZT at the auxiliary winding L2 is input to a zero-crossing detection (ZT) terminal of the control circuit 200r. A zero current detection circuit 204 detects, based on the voltage VZT across the auxiliary winding Lz, a state in which the coil current IL that flows through the inductor L1 becomes zero (zero-crossing point). In this state, the zero current detection circuit 204 asserts a zero-crossing detection signal S3.
When the zero-crossing detection signal S3 is asserted, the logic circuit 206 switches the pulse signal S2 to an on level (e.g., high level) corresponding to the on state of the switching transistor M1. The driver 208 turns on the switching transistor M1 according to the pulse signal S2.
The control circuit 200r repeats the aforementioned operation. The load current ILED is obtained by smoothing the coil current IL by means of a smoothing capacitor C1. With such an arrangement, the target current value IREF is represented by ILIM/2.
As shown in FIG. 2, immediately after the output pulse signal SOUT of the driver 208 transits to the on level, there is a great sudden increase in the current detection signal VCS due to surge noise. In order to prevent the output (limit current detection signal) S1 of the current limit comparator 202 from being asserted before the coil current IL reaches the limit current ILIM, a mask time TMSK having a predetermined length is set immediately after the switching transistor M1 is turned on. During the mask time TMSK, the comparison result obtained by the current limit comparator 202 is ignored. This operation is also referred to as “leading edge blanking (LEB)”.
Known methods that can be employed by the switching converter 100r for controlling the luminance (light amount) of the LED light source 502 include an analog dimming method and a PWM dimming method. In the analog dimming method, the current value of the load current ILED is adjusted by adjusting the target voltage VADIM so as to adjust the luminance.
In the PWM dimming method, the load current ILED is switched on and off with a variable duty ratio. Specifically, a dimming pulse S4 having a duty ratio that corresponds to the luminance is input to a PWM terminal. The dimming pulse S4 has a frequency that is sufficiently lower than that of a gate pulse SOUT of the switching transistor M1. During a period in which the dimming pulse S4 is set to high level, the switching transistor M1 performs a switching operation according to the limit current detection signal S1 and the zero-crossing detection signal S3. In this period, the LED light source 502 emits light with a luminance that corresponds to the load current ILED. During a period in which the dimming pulse S4 is set to low level, the switching transistor M1 suspends the switching operation. In this period, the load current ILED becomes zero, which turns off the LED light source 502. Thus, by changing the duty ratio of the dimming pulse S4 in a range between 0% and 100%, such an arrangement is capable of changing the luminance of the LED light source 502.
FIGS. 3A through 3C are waveform diagrams each showing the operation using the PWM dimming method. FIGS. 3A through 3C each show the waveforms of a dimming pulse S4 (PWM), an output pulse SOUT, a current detection signal VCS at a CS terminal, a coil current IL, a load current ILED, and an average value ILEDAVE of the load current ILED. It should be noted that the vertical axis and the horizontal axis shown in the waveform diagrams and time charts in the present specification are expanded or reduced as appropriate for ease of understanding. Also, each waveform shown in the drawing is simplified or emphasized for ease of understanding.
First, description will be made with reference to FIG. 3A regarding an ordinary dimming operation. During a period in which the dimming pulse S4 is set to high level, a so-called soft-switching operation is performed such that the coil current IL varies between 0 and a peak ILIM. In this period, the load current ILED is represented by ILIM/2.
With the duty ratio of the dimming pulse S4 as α %, the average value ILEDAVE of the load current ILED for each cycle of the dimming pulse S4 is represented by the following Expression. Such an arrangement is capable of changing the average value ILEDAVE in a substantially linear manner with respect to the duty ratio of the dimming pulse.ILEDAVE=ILED×α/100.
The present inventors have investigated such a PWM dimming operation, and have come to recognize the following two problems. First, description will be made with reference to FIG. 3B regarding the first problem. It should be noted that there is a difference in the time scale between FIGS. 3A and 3B.
FIG. 3B shows a state in which the dimming pulse S4 has a duty ratio of almost 100%. Description will be made directing attention to the final pulse X of the output pulses SOUT in a given cycle of the dimming pulse S4.
The dimming pulse S4 transits to low level in a period in which the final pulse X is set to high level. After a very short low level period TOFF, the dimming pulse S4 returns to high level again. In a case in which the dimming pulse S4 has such a short low level period TOFF, the coil current IL cannot drop to zero within the low level period TOFF. In this case, a next pulse Y occurs before the coil current IL drops to zero. This leads to a problem in that the switching transistor M1 turns on in a state in which the coil current IL is larger than zero, which is so-called hard switching, resulting in deviation from a quasi-resonant (QR) mode. This means that the load current ILED deviates from ½ of the peak ILIM of the coil current IL, which means degradation of the precision of the analog dimming operation. The above is the first problem.
Next, description will be made with reference to FIG. 3C regarding the second problem. FIG. 3C shows the waveforms in a case in which the dimming pulse S4 transits to low level in a state in which a given pulse Z of the output pulse SOUT is set to high level before the coil current IL reaches the peak ILIM. In this case, the final pulse Z has a higher frequency than those of the pulses generated before it. Such a change in frequency functions as noise depending on an application employing the switching converter 100r. 
The aforementioned problems are characteristic problems of a circuit configuration in which a single switching transistor is shared by the PWM dimming switching operation and the DC/DC converter switching operation. It should be noted that the above-described problems are by no means within the scope of common and general knowledge in the field of the present invention. Furthermore, it can be said that the present inventor has been the first to arrive at these problems.